Saving energy and cost in the powertrain and reducing pollution from exhaust gas and noise emissions are current goals in vehicle development. In suitable driving situations, vehicles can be temporarily advanced without being driven in a rolling or coasting mode. When a motor vehicle is in rolling mode, the internal combustion engine can operate with minimized consumption and low emissions while idling as the vehicle rolls with a disengaged drive train. If the internal combustion engine is turned off in such a driving situation to further increase the savings effect, the vehicle moves in a so-called coasting mode. These functions are already known.
Depending on the available drive train components and transmission design, the flow of power can be interrupted by shifting the transmission into neutral and/or by disengaging the start-off clutch once rolling or coasting is possible according to the above definition of terms. However, it has been shown that the known methods for identifying suitable driving situations for rolling or coasting mode do not always correctly interpret the driving situation, or are relatively complex and error-prone due to a somewhat unfiltered consideration of topographical conditions of the roadway, and/or an insufficient consideration of the current driving status. This frequently results in unnecessary shifting into neutral or interruption of the flow of power in the drive train that are undesirable depending on the situation and possibly counterproductive.
Furthermore, it is known that modern vehicles are increasingly being equipped with automatic vehicle speed control functions or vehicle speed functions and separation distance control functions for reasons of comfort, traffic, and vehicle safety. Such systems are, for example, known as Tempomat or Bremsomat systems, which maintain a desired speed as specified by the driver. Known electronic devices are available as expansions of these systems for passenger cars and commercial vehicles as ACC (adaptive cruise control) or ADR (automatic distance regulation), which create a monitoring area in front of or around the vehicle with the assistance of sensors such as radar, infrared, video or ultrasound, and with their knowledge at least the separation distance from the preceding vehicle is adapted by means of a Bremsomat by automatically letting up on gas or braking.
Known Tempomat and/or Bremsomat systems in commercial vehicles react by braking with an available auxiliary brake system, when deceleration is required on a downhill gradient and/or to adapt the separation distance from the preceding vehicle, such as an input-side or output-side hydrodynamic or electrodynamic retarder and/or the service break, where the retarder is preferably used.
A method is known from DE 102 21 701 A1 to control a rolling or coasting function for a motor vehicle with an automatic transmission. With the known method, a quantity representing the vehicle speed, brake actuation and/or actuation of a fuel supply measuring element and an operating state of the internal combustion engine are detected. A clutch device is disengaged, thereby interrupting the flow of force in the drive train when neither the gas pedal, nor the fuel supply measuring element, nor the brake pedal are actuated while the internal combustion engine is running and a specific vehicle speed is exceeded. After the clutch is disengaged, the transmission is shifted into neutral and the vehicle is therefore in a rolling mode. When the gas pedal or the brake pedal is actuated, or variables having like effect are changed, the clutch is re-engaged, thereby terminating the rolling mode. Before the clutch is engaged, the rotational speeds of the internal combustion engine and a transmission input shaft are first synchronized to return from rolling mode to regular drive as smoothly as possible. The disengagement of the clutch, i.e., rolling mode are prevented when a probable need for braking is assumed. This can for example be the case while traveling downhill, when the gas pedal is suddenly released, or when a sporty driving program is selected.
A disadvantage is that although unnecessary neutral shifting is sometimes prevented close to the situation, there is no anticipatory activation of the rolling or coasting function in particularly suitable situations or anticipatory deactivation that sufficiently considers the respective topographical conditions. In addition, the combination of rolling or coasting operation with Tempomat and Bremsomat systems, which are increasingly found in modern vehicles, is not provided.
A method is known from DE 10 2007 001 936 A1 to control a shifting clutch of an automatic transmission of a motor vehicle in which hill detection and neutral idle controls are continuously evaluated and checked. The method is executed in order to save fuel in a neutral transmission position during idling states of the internal combustion engine, while stopping at a signal, or in stop-and-go operation. In addition, the vehicle is prevented from rolling backward on an incline when there is not enough brake pressure to hold the vehicle when the transmission shifts into neutral.
Certain initial conditions have to be satisfied in such a neutral idle control system, that is, situation-controlled neutral shifting, which includes the evaluation of a hill detection flag and a hill counter. A hill detection flag can be set when there is a corresponding evaluation using hill detection, or a hill counter can be incremented. This is the case when the transmission is in a forward gear, the rotational speed of the transmission output shaft is greater than a specified output shaft rotational speed limit, a throttle position of the internal combustion engine is less than a specified throttle position limit, a brake status indicates that the vehicle brake is actuated, and the transmission temperature is within a permissible range.
Rollback-free neutral idle shifting is possible when the current counter reading is less than a specified counter limit, or when the brake pressure is greater than a precalculated brake pressure limit when the counter reading is above this counter limit, or if the brake pressure is not available and the counter reading is greater than the counter limit when a hill detection flag is set.
The known method uses continuously detected transmission, engine and brake data from the vehicle that, after being evaluated by a counter, supply reliable signals for usefully shifting the vehicle into neutral. However, it is somewhat limited to an immobile or nearly immobile vehicle and hence cannot be easily transferred in driving mode to so-called rolling or coasting on flat areas or a downhill gradient. In particular, suitable or unsuitable rolling situations on downhill grades or tapering downhill grades hence tend not to be identified, and neutral shifting is not triggered or is prevented in these situations.
DE 10 2004 017 115 A1 makes known a method for the automatic driving speed control or driving speed and separation distance control for a vehicle comprising an automatic or automated transmission, in which the driving speed can vary within the range of preselected target speeds. In that particular case, an acceleration phase is followed by a roll-out phase in which the flow of power in the drive train can be interrupted to save fuel. Topographical data, data from a monitoring device pertaining to the immediate vehicle surroundings, and/or certain vehicle parameters can be taken into account via a navigation device in the sequence of acceleration and roll-out phases. The acceleration phases can be supported by an electric motor which is present in addition to an internal combustion engine.
The known method uses a coasting or rolling mode to control speed in acceleration and roll-out phases that alternate within a Tempomat or Bremsomat system. This is also not easily applicable to downhill travel with a subsequent transition to a flat area.
WO 2003/037672 A1 makes known a multi-speed transmission, in particular for heavy commercial vehicles, which is shifted into neutral, i.e. into a rolling mode, when a low-consumption driving state is detected. A low-consumption driving state is detected and the neutral position is attained via shifting when a target speed is specified and the driving speed would decrease without the rolling mode. This is assumed, in particular, when neither the gas pedal nor the brake pedal is pressed and equivalent variables do not change. A supplemental brake system, which can be requested in rolling mode, is provided for safety reasons. The gear that is currently engaged is also taken into consideration.
The disadvantage thereof is that an anticipatory evaluation of the rolling mode is not provided, and therefore unfavorable neutral shiftings can be triggered. In particular, the topographical driving situation is not taken into sufficient consideration.
In addition, a method is known from WO 2009/037167 A1 for controlling an automated or automatic multi-step transmission of a motor vehicle, such as a heavy commercial vehicle, in which a higher gear is shifted into before the downhill gradient taper or before starting traction mode, that is, a thrust upshift is executed, to reduce fuel consumption while driving down a gradient and then transitioning into a gradient taper instead of only upshifting after leveling out or transitioning from thrust mode to traction mode as usual.
The current downhill roadway gradient is determined in a specified interval, and a function is activated when driving downhill to identify the downhill gradient taper to automatically upshift as early as possible. The rotational speed of the drive motor and possibly of an active constant braking device is thereby reduced which could cause the motor vehicle to accelerate due to the lower engine drag torque and possibly lower retarder braking torque, thereby causing the traction mode to start comparatively later. The higher gear is selected so that the motor vehicle only accelerates slightly, and the driver can control the situation safely at all times.
The downhill gradient taper is identified when the downhill gradient, just traveled, decreases steadily over a specific number of intervals and falls below a threshold. In addition, various criteria can be provided such as the actuation of the service brake, excessive vehicle acceleration, an excessively steep downhill gradient, a particularly heavy load, the operation of an auxiliary brake at its load limit, exceeding a target speed or falling below a safe separation distance regulated by an active Tempomat or Bremsomat that, for reasons of safety, lead to the blocking of thrust upshifting.
Such driving with flywheel momentum with thrust upshifting is characterized by reduced fuel consumption in commercial vehicles, especially on long stretches with a corresponding topography. However, the efficiency of a rolling or coasting function tends not to be realized in the known method.